This seems like a really specific request. I'm not sure what aspect you're looking for. But for starters, here's an interesting wikipedia page for governors that went outside the major parties:
https://en.wikipedia.org/wiki/List_of_third_party_and_indepe...
Looks like the most common thing in recent times is to just use the label "independent" rather than start a new party.
The person in question really did start a new party that was not coupled with any of the existing parties and won the capital state with crushing majority.
That is a sign of a working democracy in my book. I am not sure if US is one of the better version of a well functioning democracy given its two party system. I recall that it was one of the things that the founding fathers were afraid of when they were debating the constitution.
The founding fathers were afraid of democracy itself and put in many explicit constraints on it, never mind undesired self-organization like the two parties. So it's easy to find perspectives from which the US isn't the archytype democracy. But what's the point? That it somehow a bit farther down the road to authoritarianism as a result? I'm not sure about that.
If you mean the voters joined the new party apart from the election, by registering as a member. Then that happens in some states too. In many states voters don't even register for a party. In many others, the majority or plurality are registered as independent.
https://en.wikipedia.org/wiki/Reform_Party_of_the_United_Sta...
As for what the two parties, they are pretty entrenched, but not 100 percent given the examples I noted. But also note that both are broad coalitions, not under any small group or individual's control. Rather than having completely separate parties that operate together as a coalition in parliamentary systems. It amounts to basically the same effect. The Democratic party is a coalition of Liberals, progressives, certain immigrant and minority groups, unions, and centrists. The Republicans are a smaller coalition of conservative christians, neocons, moderates. There's some other factions in both. Neocons for example, are essentially a group that switched parties, considered liberals who side with conservatives on foreign policy. In any given election some faction might gain the advantage and win with their nominee. Meanwhile a large chunk of the country identifies as "independent" of these parties. Also note that primaries are essentially democratic too, with the nominee having to win votes to gain the nomination. Not smoke-filled backroom meetings or something. In some states, anyone can vote in a party's primary.
If it was a one-party system, then clearly not democratic, sure. But a two-party system isn't such a clear problem to me. More of a process difference.
AOC gave a long interview on how Pelosi actively tries to stifle the more left views.
You are portraying it as some happy harmony. It's far from it.